Balun circuit arrangement

ABSTRACT

Balun circuit arrangement with a balanced port side having two balanced terminals and an unbalanced port side having an unbalanced terminal. A first series connection of a first inductive impedance element (L 1 ) and a first capacitive impedance element (C 1 ) is present between a negative terminal of the two balanced terminals and the unbalanced terminal. A second series connection of a second inductive impedance element (L 2 ) and a third inductive impedance element (L 3 ) is present between a positive terminal of the two balanced terminals and the unbalanced terminal, and a second capacitive element (C 2 ) is connected between a node connecting the second and third inductive element and a ground connection. The balun circuit arrangement may be used in a combination with an oscillator circuit and a single ended antenna.

TECHNICAL FIELD

The present invention relates to a balun circuit arrangement comprisinga balanced port side having two balanced terminals and an unbalancedport side having an unbalanced terminal. In a further aspect, thepresent invention relates to a combination of an oscillator circuit anda balun circuit arrangement according to any one of the presentinvention embodiments, wherein the oscillator circuit provides an outputsignal at an output frequency and is connected to the balanced port sideof the balun circuit arrangement.

BACKGROUND ART

In many RF communication applications, such as applications according tothe ZigBee standard, use is made of a direct up-conversion transmitterarchitecture wherein a local oscillator is operating at twice the outputfrequency (operational transmitting frequency). This type of transmitterarchitecture is susceptible to oscillator injection pulling (oroscillator re-modulation), which is an unwanted effect in RF design. Thearticle by B. Razavi, ‘A Study of Injection Locking and Pulling inOscillators’, IEEE Journal of Solid-State Circuits, vol. 39, no. 9,September 2004, pp. 1415-1424, describes this phenomenon.

A possible solution to mitigate the effects of injection pulling wouldbe to shift operation of the local oscillator to four times or twothirds of the output frequency, but this will result in increased powerconsumption and/or increased silicon area (if operating at two thirds ofthe output frequency).

SUMMARY OF INVENTION

The present invention seeks to provide an effective solution to mitigatethe effects of injection pulling, without negative effects on cost andoverall performance of the RF communication application.

According to the present invention, a balun circuit arrangementaccording to the preamble defined above is provided, wherein the baluncircuit arrangement further comprises a first series connection of afirst inductive impedance element and a first capacitive impedanceelement between a negative terminal of the two balanced terminals andthe unbalanced terminal, a second series connection of a secondinductive impedance element and a third inductive impedance elementbetween a positive terminal of the two balanced terminals and theunbalanced terminal, wherein a second capacitive element is connectedbetween a node connecting the second and third inductive element and aground connection. This combination and structure of components providesa combination of balun and filtering capabilities, while at the sametime mitigating injection pulling effects.

A very good effect is achieved in an embodiment wherein the firstinductive impedance element is directly connected to the negativeterminal of the two balanced terminals. If the two components in thefirst series connection are exchanged, i.e. in an embodiment wherein thefirst capacitive impedance element is directly connected to the negativeterminal of the two balanced terminals, still effects of injectionpulling are lessened.

In a further embodiment, the inductive and capacitive impedance elementscomprise discrete components, i.e. using inductors and capacitors, thusforming a lumped balun circuit arrangement. One or more of the inductiveand/or capacitive impedance elements may comprise a parallel circuit ofan inductor and a capacitor in an even further embodiment. This may beby equivalence (e.g. by parasitic impedances), or by actual parallelconnected components.

In a second aspect, the present invention relates to a combination of anoscillator circuit and a balun circuit arrangement as defined above,wherein the first inductive impedance element and the second inductiveimpedance element have a higher impedance value in a frequency regionaround a second harmonic frequency of the oscillator circuit as comparedto in a frequency region around the output frequency of the oscillatorcircuit. This will provide for an effective suppression of injectionpulling. Furthermore, the first inductive impedance element and thesecond inductive impedance element may have a peak impedance value in afrequency region around a second harmonic frequency of the oscillatorcircuit. Specific types of inductors are available that show such acharacteristic, i.e. lower impedance below and above the frequencyregion. The present combination is especially effective in an embodimentwherein the oscillator circuit has a direct up-conversion transmitterarchitecture with a local oscillator operating at twice the outputfrequency, as such an oscillator circuit is known to be susceptive forinjection pulling.

In a further embodiment, the oscillator circuit is an integrated circuitpositioned on a printed circuit board and the balun circuit arrangementis implemented on the printed circuit board, e.g. on a single layer PCB,which yields a very cost-effective implementation. Alternatively, thecombination is implemented as a single integrated circuit.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be discussed in more detail below, using anumber of exemplary embodiments, with reference to the attacheddrawings, in which

FIG. 1 shows a block diagram of a combination of an oscillator circuit,a balun circuit arrangement and antenna according to an embodiment ofthe present invention.

FIG. 2 shows a circuit diagram of a balun circuit arrangement accordingto an embodiment of the present invention;

FIG. 3 shows a circuit diagram of an alternative embodiment of the baluncircuit arrangement; and

FIG. 4 shows a circuit diagram of a further alternative embodiment ofthe balun circuit arrangement.

DESCRIPTION OF EMBODIMENTS

Radiofrequency (RF) communication applications are widespread, and thepresent invention embodiments relate to specific applications in lowenergy and moderate to low data rate RF communication applications, suchas ZigBee and Bluetooth applications.

FIG. 1 shows a block diagram of a common set-up of an RF communicationarrangement 7 wherein the present invention embodiments can be applied.Shown are an oscillator circuit 5 providing a balanced output signal aswell as an antenna 6 for transmitting (and receiving) RF signals. Theoscillator circuit 5 is e.g. implemented as a radio chip, such as aZigBee or Bluetooth radio chip. In many applications, the antenna 6 is asingle ended antenna, and a balun circuit arrangement 1 is used totransform the balanced (or differential) signal from the oscillatorcircuit 5 to an unbalanced signal to the antenna 6 (and vice versa). InFIG. 1 also the connecting terminals of the balun circuit arrangement 1are shown, i.e. a balanced port side having two balanced terminals 2, 3and an unbalanced port side having an unbalanced terminal 4.

In general all radio functions needed for a specific application areintegrated in a radio chip (i.e. oscillator circuit 5 of FIG. 1), whiche.g. comprises a local oscillator (e.g. a voltage controlled oscillator,VCO) and a power amplifier (PA) in the transmitter branch of the radiochip.

In direct up-conversion transmitter architectures, which are nowcommonly used in many RF communication applications, such as ZigBee andBluetooth radio chips, oscillator injection pulling may be a problem andmay cause unwanted effects such as error vector magnitude (EVM)degradation (or distortion) in the transmitter output signal. It hasbeen found that indeed injection pulling may be the root cause of theseeffects. Reference is made to the article by B. Razavi mentioned in theparagraph Background art above.

According to the present invention embodiments, a solution for theproblem of oscillator injection pulling is found by providing a properlydesigned balun circuit arrangement 1 connected to balanced output signalof the oscillator chip 5. In other words, a balun circuit arrangement 1is provided as shown in the circuit diagram of FIG. 2, comprising abalanced port side having two balanced terminals 2, 3 and an unbalancedport side having an unbalanced terminal 4. A first series connection ofa first inductive impedance element L1 and a first capacitive impedanceelement C1 is provided between a negative terminal 2 of the two balancedterminals 2, 3 and the unbalanced terminal 4, as well as a second seriesconnection of a second inductive impedance element L2 and a thirdinductive impedance element L3 between a positive terminal 3 of the twobalanced terminals 2, 3 and the unbalanced terminal 4, wherein a secondcapacitive element C2 is connected between a node b connecting thesecond and third inductive element L2, L3 and a ground connection.

By providing the first and second series connections directly connectedto the balanced terminals 2, 3 to which an oscillator circuit 5 (seeFIG. 4 below) is connected, the effects of injection pulling can beeffectively mitigated. This is due to the mainly inductive nature of thefirst and second series connection directly at the balanced terminals 2,3. Furthermore, this is accomplished with a balun circuit arrangementhaving a low part count, thus providing a very cost-efficient solution.

In the circuit diagram of FIG. 2, the balun circuit arrangement 1 alsocomprises a fourth inductive impedance element Lm connected betweennodes of the first series connection and nodes of the second seriesconnection, in this specific embodiment directly between the twobalanced terminals 2, 3. This fourth inductive element Lm is applied tocancel residue parasitic capacitance (of the connected oscillatorcircuit 5), but is also operative for a biasing function of theconnected oscillator circuit 5. The biasing function is furtherimplemented using a bias inductive impedance element Lbias connectedbetween the unbalanced terminal 4 and a bias terminal Vbias. As shownthis biasing may in the exemplary implementation as shown in FIG. 2 beaccomplished using an additional bypass capacitive impedance elementCbypass connected to ground. It is noted that the fourth inductiveimpedance Lm and bias inductive impedance element Lbias may actuallycontribute to the desired characteristics of the balun circuitarrangement 1, i.e. providing a best combination of properties relatedto balun, filtering, injection pulling mitigation and for reducing thecost of circuit components.

In the embodiment of FIG. 2, the first inductive impedance element L1 isdirectly connected to the negative terminal 2 of the two balancedterminals 2, 3. As mentioned above, the direct connection of aninductive impedance element as close as possible to the oscillatorcircuit 5 connected to the two balanced terminals 2, 3 contributes tomitigation of injection pulling effects. In the second seriesconnection, both series connected second and third inductive impedanceelements L2 and L3 contribute to this mitigating effect.

Alternatively, as shown in the alternative embodiment of FIG. 3, thefirst capacitive impedance element C1 is directly connected to thenegative terminal 2 of the two balanced terminals 2, 3. As the firstseries connection C1, L1 is still of an overall inductive nature, themitigating effect is maintained, especially with the second inductiveimpedance element L2 connected to the positive terminal 3.

In FIG. 3, the fourth inductive impedance element Lm is shown in analternative position, i.e. connected between a node a of the firstseries connection L1, C1, and a node b of the second series connectionL2, L3, C2. FIG. 4 shows yet a further alternative embodiment, whereinthe fourth inductive impedance element Lm is connected between theunbalanced terminal 4 and the first inductive impedance element L1. Notethat the electric small signal behavior of the three port balun circuitarrangement 1 is relevant for the functionality and effect of thepresent invention embodiments. The fourth inductive impedance element Lmin the FIG. 4 embodiment may thus be seen as part of the first seriesconnection between the negative terminal 2 and unbalanced terminal 4.

Furthermore, it is noted that as an alternative for the embodiments asshown in FIG. 2-4, the bias inductive impedance element Lbias (and biasvoltage terminal Vbias and additional bypass capacitive impedanceelement Cbypass) may be connected to the node a of the first seriesconnection L1, C1.

The inductive and capacitive impedance elements as described above withreference to the embodiments of FIG. 2-4 may comprise discretecomponents, i.e. inductors and capacitors. This is particularlyadvantageous when the balun circuit arrangement is implemented as alumped balun circuit on a printed circuit board (PCB) on which also theoscillator circuit 5 is positioned. By properly selecting the values ofthese components, the proper and desired characteristics of the baluncircuit arrangement 1 may be obtained.

Furthermore, one or more of the inductive and/or capacitive impedanceelements may comprise a parallel circuit of an inductor and a capacitor.This may be included in determining the balun circuit arrangementcharacteristics, by equivalence, but also when parasitic impedances arepresent, or if actual parallel components need to be used to obtain theproper element values.

The present invention embodiments as described above are functional inmitigating injection pulling of an oscillator circuit 5 connected to thebalanced port side of the balun circuit arrangement 1. When having asingle ended antenna 6 an additional benefit is provided that that asingle layer printed circuit board can be used in an actualimplementation, lowering the overall design cost requirements. Generallyspeaking single ended antennas have some advantages over balancedantennas and are used in many situations. However, if the balun circuitarrangement 1 is to be built on a single layer PCB there is a challengeof providing filtering efficiently, because single layer PCB does notsupport unbalanced signal propagation. The balun circuit arrangementaccording to the present invention embodiments can solve this problemwith its filtering functionality.

In a further aspect the present invention relates to a combination 7 ofan oscillator circuit 5 and a balun circuit arrangement 1 as describedin the embodiments above. The oscillator circuit 5 provides an outputsignal at an output frequency and is connected to the balanced port sideof the balun circuit arrangement 1. With designs based on smaller chips(oscillator circuit 5) which use lower currents and provide more outputpower, there occurs a larger dependency on pulling effects related tothe circuit chain of VCO, PA in the oscillator circuit 5 and a loadconnected to the oscillator circuit 5. E.g. the VCO design may be basedon 4.8 GHz to provide a 2.4 GHz output signal, the TX design may beimplemented with direct up-conversion. Furthermore, a small-size,low-current design is desired, as well as a larger PA output power.Further, in an actual implementation, filtering is needed around thebalun circuit arrangement function for regulatory requirements asdefined by FCC and ETSI. Complicating factor may be that with a singlelayer PCB design there is no dedicated ground plane to supportpropagation of an unbalanced signal. Therefore designers of applicationcircuits are seeking for a best combination of properties andcharacteristics related to balun, filtering, injection pullingmitigation and for reducing the cost of PCB and circuit components.

This may all be achieved in an embodiment of the present inventioncombination 7, wherein the first inductive impedance element L1 and thesecond inductive impedance element L2 (which are directly connected tothe balanced terminals 2, 3) have a higher impedance value in afrequency region around a second harmonic frequency of the oscillatorcircuit 5 as compared to in a frequency region around the output(carrier) frequency of the oscillator circuit 5. This may beaccomplished by selecting the right type and make of components formingthe first and second inductive impedance elements L1, L2. Right valueinductors, when considering their parasitics, can show parallelresonance in the vicinity of the second harmonic frequency, aiding inthe mitigation of injection pulling. The same parasitic parallelresonances of the first, second and third inductive impedance elementsL1, L2 and L3, also contribute most to the filter capacities of thebalun circuit arrangement (second harmonic is the most difficult to befiltered, because it's frequency is closest to that of the signal amongall harmonics). It is noted this filtering mechanism differs from anormal L-C ladder filter.

In alternative wording, the first inductive impedance element L1 and thesecond inductive impedance element L2 have a peak impedance value in afrequency region around a second harmonic frequency of the oscillatorcircuit 5. Specific types of inductors which are commercially availableare having this characteristic, i.e. a lower impedance below and abovethe second harmonic frequency region.

In an exemplary embodiment, inductors and capacitors are used which arecommercially available from the Murata LQG15HN series and GRM36COGseries, respectively. Exemplary values for a ZigBee related applicationthen are L1=5.6 nH; C1=0.6 pF; L2=6.2 nH; L3=6.8 nH; C2=0.6 pF; Lm=3.0nH; Lbias=22 nH and Cbypass=10 pF. In the characteristics of thesecomponents it is noted that the 6.2 nH and the 5.6 nH inductors have ahigh impedance within the second harmonic frequency range of 4810-4960MHz (ZigBee), while the inductors with either lower or higher valueshave a lower impedances in the second harmonic frequency range.

In implementations wherein the oscillator circuit 5 has a directup-conversion transmitter architecture with a local oscillator operatingat twice the output frequency, as described above for a ZigBeeapplication, the effects of oscillator injection pulling is thenmitigated.

In exemplary embodiments, the first inductive impedance element L1 andthe second inductive impedance element L2 have a value of between 4 and8 nH, e.g. 5 and 7 nH, respectively. The frequency region is e.g. from4810 to 4960 MHz which is the 2^(nd) harmonic band in case of ZigBeeapplications.

In the implementation of the present invention embodiments as describedabove with reference to FIG. 1, the combination 7 also comprises anantenna 6 connected to the unbalanced port side of the balun circuitarrangement 1, e.g. a single ended antenna like an inverted F antenna.In an embodiment, the oscillator circuit 5 may be (part of) anintegrated circuit positioned on a printed circuit board (PCB) and thebalun circuit arrangement 1 may be implemented on the (single layer)printed circuit board. As an alternative, the combination 7 of thepresent invention is implemented as a single integrated circuit.

For the present invention embodiments, it is easier to adapt the designto suit different practical situations, than that of a traditional baluncircuit arrangement, especially if a single layer PCB implementation isconsidered. A single layer PCB does not support unbalanced signalpropagation, so no filter can be connected after a traditional baluncircuit arrangement. In such a case, a traditional balun circuitarrangement has to function between the differential impedance of theoscillator circuit 5 and the single ended impedance of the antenna 6, atthe required frequency. On the other hand, the traditional balun circuitarrangement has only two component values L and C which can be selected.For this reason in general a traditional balun circuit arrangement canat best sub-optimally work for a single layer PCB implementation. Forthe present invention balun circuit arrangement 1, although it alsoneeds to cope with the same situation, it has much more freedom toadjust component values to ensure a satisfactory functioning.

The present invention embodiments have been described above withreference to a number of exemplary embodiments as shown in the drawings.Modifications and alternative implementations of some parts or elementsare possible, and are included in the scope of protection as defined inthe appended claims.

1. Balun circuit arrangement comprising: a balanced port side having twobalanced terminals and an unbalanced port side having an unbalancedterminal, a first series connection of a first inductive impedanceelement and a first capacitive impedance element between a negativeterminal of the two balanced terminals and the unbalanced terminal, asecond series connection of a second inductive impedance element and athird inductive impedance element between a positive terminal of the twobalanced terminals and the unbalanced terminal, wherein a secondcapacitive element is connected between a node connecting the second andthird inductive element and a ground connection, and a fourth impedanceelement connected between the first inductor element and the thirdinductor element.
 2. Balun circuit arrangement according to claim 1,wherein the first inductive impedance element is directly connected tothe negative terminal of the two balanced terminals.
 3. Balun circuitarrangement according to claim 1, wherein the first capacitive impedanceelement is directly connected to the negative terminal of the twobalanced terminals.
 4. Balun circuit arrangement according to claim 1,wherein the fourth inductive element is connected between nodes of thefirst series connection and nodes of the second series connection. 5.Balun circuit arrangement according to claim 1, further comprising abias inductive impedance element connected between the unbalancedterminal and a bias terminal.
 6. Balun circuit arrangement according toclaim 1, wherein the inductive and capacitive impedance elementscomprise discrete components.
 7. Balun circuit arrangement according toclaim 1, wherein one or more of the inductive and/or capacitiveimpedance elements comprise a parallel circuit of an inductor and acapacitor.
 8. Balun circuit arrangement according to claim 1, whereinthe balun circuit arrangement is combined with an oscillator circuit,and wherein the oscillator circuit provides an output signal at anoutput frequency and is connected to the balanced port side of the baluncircuit arrangement, and the first inductive impedance element and thesecond inductive impedance element have a higher impedance value in afrequency region around a second harmonic frequency of the oscillatorcircuit as compared to in a frequency region around the output frequencyof the oscillator circuit.
 9. Balun circuit arrangement according toclaim 8, wherein the first inductive impedance element and the secondinductive impedance element have a peak impedance value in a frequencyregion around a second harmonic frequency of the oscillator circuit. 10.Balun circuit arrangement according to claim 8, wherein the oscillatorcircuit has a local oscillator operating at twice the output frequency.11. Balun circuit arrangement according to claim 8, wherein the firstinductive impedance element and the second inductive impedance elementhave a value of between 4 and 8 nH, e.g. 5 and 7 nH, respectively. 12.Balun circuit arrangement according to claim 8, wherein the frequencyregion is from 4810 to 4960 MHz.
 13. Balun circuit arrangement accordingto claim 8, further comprising an antenna connected to the unbalancedport side of the balun circuit arrangement.
 14. Balun circuitarrangement according to claim 8, wherein the oscillator circuit is anintegrated circuit positioned on a printed circuit board and the baluncircuit arrangement is implemented on the printed circuit board. 15.Balun circuit arrangement according to claim 8, wherein the combinationis implemented as a single integrated circuit.